What did Han Wudi do to the Great Wall?

He launched several wars against the Huns during 127BC-119BC. The construction was conducted for basically 4 times during this period. According to history record, the first time was on 127BC when the General Wei Qing and his troop defeated the Loufan tribe. It was a renovation on the previous walls built in the Qin Dynasty. The second time was in 121BC when the general Huo Qubing won the fight and get back Hexi area. To enhance the security of this area, Han Wudi ordered to built the wall from present Yongdeng County - Jiuquan City, Gansu Province. The thirds large scale of construction was conducted during 111BC-110BC from Jiuquan to Yumenguan Pass. The forth time was Yumenguan Pass to Luobupo area in Xinjiang Province. The wall stretched into the west inland area of China and it was the main part of Han's wall. Those walls didn't play important role in the wars against the Huns. but a barrer to protect in case of an intruding from then on.